The invention concerns a process for the production of zeolite membranes which are supported on porous glass, and membranes so produced.
Zeolites are used for their properties of separation by selective adsorption or for their catalytic properties. However, separation using powdered zeolite is a batch process. A zeolite membrane offers the possibility of using a continuous process of economical interest for separating molecules.
A variety of zeolite membrane production processes have already been described. Such membranes are frequently prepared by crystallising a film of zeolite on and/or in a porous support or on a non porous support. There are two possible cases.
In the first case, the support is immersed in a zeolite precursor gel and the ensemble is subjected to hydrothermal conditions to crystallise the zeolite. Japanese patent application JP-A-60/129119, for example, describes a membrane composed of a film of zeolite crystallised on the surface of an alumina support. In U.S. Pat. No. 5,100,596, therefor a membrane is produced by crystallising a molecular sieve on a non porous surface and separating the film from that surface. In European patent EP-A-0 481 660, which concerns a supported zeolite membrane, the zeolite layer is directly bonded to the support. Before crystallising the zeolite, the support is brought into contact with a solution or suspension containing a source of silicic acid to form silicic acid oligomers on the surface. Japanese patent application JP-A-06/321530 describes a process for the production of composite membranes in which the synthesis temperature is increased during crystallisation.
In the second case, the support is brought into contact with a colloidal solution, separated therefrom and placed in saturated steam to crystallise the zeolite from the adsorbed solution. International patent application WO-A-93/17781 describes a process for forming a zeolite membrane by exposing an aqueous or alcoholic colloidal solution deposited on a porous support to saturated steam.
Adhesion of the zeolite to the support must be good in a composite membrane. As a result, the zeolite film must be bonded directly to the support. Treatment of the support surface to increase its reactivity is sometimes necessary. The reactivity can be increased, for example, by forming silicic acid oligomers on the support surface using external reactants. As a result, bonding of these oligomers to the support is decidedly weak and their distribution is not homogeneous.